null point exception in array lists

[c_walker]

Hi, I am really new in Java. I need help regarding NullPointerExceptions.
The class compiled but when I riun my program, it says that I have a null point exception. What do you think are my mistakes? Thanks in advance.

Here are my codes(all from one class):

Instance fields

    private double earnings;
    ArrayList<Copier> copierList;
    ArrayList<DepartmentInvoice> departmentList;
    String[] sizeList = {"Letter","Legal","A4","A3"};
    ArrayList<String> deptnames = new ArrayList<String>();

Constructor:

public CopyOffice()
    {
        ArrayList<Copier> copierList = new ArrayList<Copier>();
        ArrayList<DepartmentInvoice> departmentList = new ArrayList<DepartmentInvoice>();
        earnings = 0.0;
    }

addDepartment method

 /**
   * The addDepartment method calls the findDept method to check whether a department is already in the ArrayList.
   * If it does not exist, an instance of DepartmentInvoice is created and stored in the ArrayList. 
   * A message is returned to indicate if the department is added or if the department already exists. 
   * 
   * @param  name  the unique name of the department
   * @return the message about the success or failure of adding a Department
   */
  
    public String addDepartment(String name)
    {
        String message="";
        if (deptnames.isEmpty())
        {
            deptnames.add(name);
            [COLOR="Red"]departmentList.add(new DepartmentInvoice(name));[/COLOR]----->[B]null pointer exception: null[/B]
            message = "Department added: " + name;
        }
        else
        {
            if (findDept(name)== null)
            {
                   
                   departmentList.add(new DepartmentInvoice(name));
                   message = "Department added: " + name;
            }               
            else
                message= "Department already exists: " + name;       
        }
                   
        return message;     
    }

findDept method

/**
  * The findDept method of class DepartmentInvoice returns the DepartmentInvoice record. 
  * The method returns null when no record is found. 
  * 
  * @ param  s   the name of the department being searched 
  * @ return the DepartmentInvoice record being searched
  */ 
 
   public DepartmentInvoice findDept( String s )
   {
       DepartmentInvoice a = null;
       for( int i = 0; i < departmentList.size(); i++ )
       {
           String j = departmentList.get( i ).getName();
           if( j.equalsIgnoreCase( s ))
           {
               a = departmentList.get( i );
               break;
           }
       }
       return a;

[Fubarable]

Your problem is here:

import java.util.ArrayList;

public class Foo2 {
   ArrayList<String> list1;
   ArrayList<String> list2;
   
   public Foo2() {
      ArrayList<String> list1 = new ArrayList<String>();
      list2 = new ArrayList<String>();
   }
   
   public void useList() {
      
      // This works:
      list2.add("new String");
      
      
      // but this doesn't:
      list1.add("new String");  // know why?
   }
}

list2 works, but list1 doesn't. Do you see why? The question is what gets initialized in the constructor? The field that was declared in the class at the top, or a new variable that is declared in (and only visible from) the constructor?

Make sense?

Oh, and before I forget -- welcome to the forum!

[c_walker]

It works. Thank you!

[Fubarable]

You're quite welcome!